1. Field of the Invention
The present invention relates to an unmanned or automatic carrier suitably used for carrying semiconductor wafers or the like.
2. Description of the Related Art
Recently, in factories for manufacturing semiconductor devices, manufacturing equipment has become automated, and simultaneously, carriers for carrying the semiconductor devices from one manufacturing equipment to another manufacturing equipment have been automated, i.e., unmanned.
A first prior art automatic carrier includes a body, a driving wheel mounted at a central portion of the body, and driven wheels mounted at front and back sides of the body. In this case, each of the driven wheels is formed by a caster whose direction of motion can smoothly follow the direction of the carrier.
In the first prior art automatic carrier, however, when the carrier is expected to reverse its direction of motion, the casters fluctuate greatly so as to vibrate the entire carrier. As a result, semiconductor wafers or the like mounted on the carrier may be broken. Also, when the carrier is heavy, the pivoting movement of the casters may damage guide tapes and position detection marks adhered to the ground.
In order to overcome the above-mentioned defects of the first prior art automatic carrier, a second prior art automatic carrier has suggested that the casters be mounted via direct acting guides and direct acting rails to the body (see JP-A-HEI4-310403). This will be explained later in detail.
In the second prior art automatic carrier, however, when the carrier is expected to turn by changing its direction of motion over a large angle such as 90.degree., the impact of the direct acting guides against the stoppers of the direct acting rails becomes large, so that the entire carrier is greatly vibrated. As a result, semiconductor wafers or the like mounted on the carrier may be broken. In addition, the sliding movement of the direct acting rails along the direct acting guides invites mechanical friction and looseness therebetween, thus vibrating the entire carrier.